Method and system for preventing fouling of surfaces

ABSTRACT

A method of anti-fouling of a surface while said surface is at least partially submersed in an liquid environment, comprising: providing an anti-fouling light ( 9 ); distributing at least part of the light ( 9 ) through an optical medium ( 5 ) comprising a silicone material and/or UV grade fused silica; emitting the anti-fouling light ( 9 ) from the optical medium ( 5 ) and from the surface.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§371 of International Application No. PCT/IB2014/061579, filed on May21, 2014, which claims the benefit of U.S. Provisional Application61/826,148, filed on May 22, 2013 and European Patent Application No.13191713.0, filed on Nov. 6, 2013. These applications are herebyincorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to methods for preventing fouling, orcommonly referred to as anti-fouling, of surfaces and to devices forperforming these methods. The disclosure specifically relates to methodsand devices for anti-fouling of the hull of ships.

BACKGROUND

Biofouling or biological fouling is the accumulation of microorganisms,plants, algae, and/or animals on surfaces. The variety among biofoulingorganisms is highly diverse and extends far beyond attachment ofbarnacles and seaweeds. According to some estimates, over 1700 speciescomprising over 4000 organisms are responsible for biofouling.Biofouling is divided into microfouling which includes biofilm formationand bacterial adhesion, and macrofouling which is the attachment oflarger organisms. Due to the distinct chemistry and biology thatdetermine what prevents organisms from settling, these organisms arealso classified as hard or soft fouling types. Calcareous (hard) foulingorganisms include barnacles, encrusting bryozoans, mollusks, polychaeteand other tube worms, and zebra mussels. Examples of non-calcareous(soft) fouling organisms are seaweed, hydroids, algae and biofilm“slime”. Together, these organisms form a fouling community.

In several circumstances biofouling creates substantial problems.Machinery stops working, water inlets get clogged, and hulls of shipssuffer from increased drag. Hence the topic of anti-fouling, i.e. theprocess of removing or preventing fouling from forming, is well known.In industrial processes, bio-dispersants can be used to controlbiofouling. In less controlled environments, organisms are killed orrepelled with coatings using biocides, thermal treatments or pulses ofenergy. Nontoxic mechanical strategies that prevent organisms fromattaching include choosing a material or coating with a slipperysurface, or creation of nanoscale surface topologies similar to the skinof sharks and dolphins which only offer poor anchor points.

SUMMARY

Biofouling on the hull of ships, as illustrated in FIG. 1, causes asevere increase in drag, and thus increased fuel consumption. It isestimated that an increase of up to 40% in fuel consumption can beattributed to biofouling. As large oil tankers or container transportships can consume up to

200.000 a day in fuel, substantial savings are possible with aneffective method of anti-biofouling.

Herewith an approach is presented based on optical methods, inparticular using ultra-violet light (UV). It is well-known that mostmicro-organisms are killed, rendered inactive or unable to reproducewith ‘sufficient’ UV light. This effect is mainly governed by the totaldose of UV light. A typical dose to kill 90% of a certain micro-organismis 10 mW-hours per square meter, details are contained in the followingparagraphs regarding UV light, and the associated Figures.

Ultraviolet Light in General

Ultraviolet (UV) is that part of electromagnetic light bounded by thelower wavelength extreme of the visible spectrum and the X-ray radiationband. The spectral range of UV light is, by definition between 100 and400 nm (1 nm=10⁻⁹ m) and is invisible to human eyes. Using the CIEclassification the UV spectrum is subdivided into three bands:

UVA (long-wave) from 315 to 400 nm

UVB (medium-wave) from 280 to 315 nm

UVC (short-wave) from 100 to 280 nm

In reality many photobiologists often speak of skin effects resultingfrom UV exposure as the weighted effect of wavelength above and below320 nm, hence offering an alternative definition.

A strong germicidal effect is provided by the light in the short-waveUVC band. In addition erythema (reddening of the skin) andconjunctivitis (inflammation of the mucous membranes of the eye) canalso be caused by this form of light. Because of this, when germicidalUV-light lamps are used, it is important to design systems to excludeUVC leakage and so avoid these effects. In case of immersed lightsources, absorption of UV light by water may be strong enough that UVCleaking is no problem for humans above the liquid surface.

Self evidently people should avoid exposure to UVC. Fortunately this isrelatively simple, because it is absorbed by most products, and evenstandard flat glass absorbs substantially all UVC. Exceptions are e.g.quartz and PTFE (PolyTetraFluorEth(yl)ene). Again fortuitously, UVC ismostly absorbed by dead skin, so erythema can be limited. In additionUVC does not penetrate the eye's lens; nevertheless, conjunctivitis canoccur and though temporary, it is extremely painful; the same is true oferythemal effects.

Where exposure to UVC light occurs, care should be taken not to exceedthe threshold level norm. FIG. 2 shows these values for most of the CIEUV spectrum. In practical terms, Table 1 gives the American Congress ofGovernmental and Industrial Hygienist's (ACGIH) UV Threshold LimitEffective Irradiance Values for human exposure related to time. At thistime it is worth noting that radiation wavelengths below 240 nm formsozone, O₃, from oxygen in air. Ozone is toxic and highly reactive; henceprecautions have to be taken to avoid exposure to humans and certainmaterials.

TABLE 1 permissible UVC exposures for humans according to ACGIH Durationof exposure per day Irradiance (μW/cm²) 8 hours 0.2 4 hours 0.4 2 hours0.8 1 hour 1.7 30 minutes 3.3 15 minutes 6.6 10 minutes 10 5 minutes 201 minute 100Generation and Characteristics of Short-Wave UV Light

The most efficient source for generating UVC is the low-pressure mercurydischarge lamp, where on average 35% of input watts is converted to UVCwatts. The radiation is generated almost exclusively at 254 nm viz. at85% of the maximum germicidal effect (FIG. 3). Philips' low pressuretubular flourescent ultraviolet (TUV) lamps have an envelope of specialglass that filters out ozone-forming radiation, in this case the 185 nmmercury line. The spectral transmission of this glass is shown in FIG. 4and the spectral power distribution of these TUV lamps is given in FIG.5.

For various Philips germicidal TUV lamps the electrical and mechanicalproperties are identical to their lighting equivalents for visiblelight. This allows them to be operated in the same way i.e. using anelectronic or magnetic ballast/starter circuit. As with all low pressurelamps, there is a relationship between lamp operating temperature andoutput. In low pressure lamps the resonance line at 254 nm is strongestat a certain mercury vapour pressure in the discharge tube. Thispressure is determined by the operating temperature and optimises at atube wall temperature of 40° C., corresponding with an ambienttemperature of about 25° C. It should also be recognised that lampoutput is affected by air currents (forced or natural) across the lamp,the so called chill factor. The reader should note that, for some lamps,increasing the air flow and/or decreasing the temperature can increasethe germicidal output. This is met in high output (HO) lamps viz. lampswith higher wattage than normal for their linear dimension.

A second type of UV source is the medium pressure mercury lamp, here thehigher pressure excites more energy levels producing more spectral linesand a continuum (recombined radiation) (FIG. 6). It should be noted thatthe quartz envelope transmits below 240 nm so ozone can be formed fromair. Advantages of medium pressure sources are:

-   -   high power density;    -   high power, resulting in fewer lamps than low pressure types        being used in the same application; and    -   less sensitivity to environment temperature.        The lamps should be operated so that the wall temperature lies        between 600 and 900° C. and the pinch does not exceed 350° C.        These lamps can be dimmed, as can low pressure lamps.

Further, Dielectric Barrier Discharge (DBD) lamps can be used. Theselamps can provide very powerful UV light at various wavelengths and athigh electrical-to-optical power efficiencies.

The germicidal doses listed above can also easily be achieved withexisting low cost, lower power UV LEDs. LEDs can generally be includedin relatively smaller packages and consume less power than other typesof light sources. LEDs can be manufactured to emit (UV) light of variousdesired wavelengths and their operating parameters, most notably theoutput power, can be controlled to a high degree.

An basic idea underlying the present disclosure is to cover significantamounts of a protected surface to be kept clean from fouling, preferablythe entire protected surface, e.g. the hull of a ship, with a layer thatemits germicidal light, in particular UV light.

Accordingly, herewith a method of anti-fouling of a protected surface aswell as a lighting module and a system for anti-fouling of a protectedsurface according to the appended claims are provided.

A method comprises providing anti-fouling light and emitting theanti-fouling light in a direction away from a protected surface, whereinat least part of the light is distributed across a substantial part ofthe protected surface by an optical mediumbefore being emitted in thedirection away from the protected surface. In embodiments, the themethod comprises emitting the anti-fouling light from a substantiallyplanar emission surface of the optical medium. In embodiments the methoduses a light guide to distribute the light across a substantial part ofthe protected surface and comprises silicone material and/or UV gradesilica material, in particular quartz. The method is preferably executedwhile the protected surface is at least partially submersed in a liquidenvironment.

A lighting module for anti-fouling of a protected surface comprises atleast one light source for generating anti-fouling light and an opticalmedium for distributing the anti-fouling light from the light source.The at least one light source and/or the optical medium may be at leastpartly arranged in, on and/or near the protected surface so as to emitthe anti-fouling light in a direction away from the protected surface.The lighting module is adapted to preferably emit the anti-fouling lightwhile the protected surface is at least partially submersed in an liquidenvironment. In an embodiment, the optical medium is a light guidecomprises a silicone material and/or UV grade silica material.

The lighting module for anti-fouling of a protected surface may also beprovided as a foil for applying to the protected surface, the foilcomprising at least one light source for generating anti-fouling lightand a sheet-like optical medium for distributing the anti-fouling lightacross the foil. In embodiments the foil has a thickness in an order ofmagnitude of a couple of millimeters to a few centimeters. Inembodiments, the foil is not substantially limited in any directionperpendicular to the thickness direction so as to provide substantiallylarge foil having sizes in the order of magnitude of tens or hundreds ofsquare meters. The foil may be substantially size-limited in twoorthogonal directions perpendicular to the thickness direction of thefoil, so as to provide an anti-fouling tile; in another embodiment thefoil is substantially size-limited in only one one directionperpendicular to a thickness direction of the foil, so as to provide anelongated strip of anti-fouling foil.

The lighting module, whether arranged in, on and/or near the protectedsurface or whether provided as a separate foil, comprises an emissionsurface for emitting the anti-fouling light from the optical medium intoan environment and a application surface, opposed the emission surface,for applying or arranging the lighting module to the protected surface.In a preferred embodiment the emission surface of the light module issubstantially planar so as to avoid pits and indent which may becomeseeds of fouling and so as to avoid bulges to limit the amount of dragcaused by the structure when applied to the protected surface. Theadvantage of a substantially planar surface versus a surface comprisingindents and bulges or having a substantial surface roughness is that itwill be more difficult for microorganisms to adhere to a substantialplane surface, especially in combination with drag effects in a liquidenvironment, than they would onto a rough surface or into pits comprisesin said surface. The term ‘substantially planar’ emission surface hereinrefers to a surface masking or obscuring the thickness of light sourcesand wiring connections embed in or attached to the lighting module. Theterm ‘substantially planar’ may also refer to masking or obscuring someconstructional uneveness of the protected surface, thereby evenimproving the drag properties of the protected surface in the liquidenvironment. Example of constructional uneveness of the protectedsurface are welds, rivets, etc. The term ‘substantially planar’ can bequantified as resulting in variations in the average thickness of thelight modules of less than 25%, preferably less than 10%. ‘Substantiallyplanar’ therefore not necessarily requires a surface roughness of amachined surface finish.

In a preferred embodiment the lighting module comprises atwo-dimensional grid of light sources for generating anti-fouling lightand the optical medium is arranged to distribute at least part of theanti-fouling light from the two-dimensional grid of light sources acrossthe optical medium so as to provide a two-dimensional distribution ofanti-fouling light exiting the light emitting surface of the lightmodule. The two-dimensional grid of light sources may be arranged in achickenwire structure, a close-packed structure, a rows/columnsstructure, or any other suitable regular or irregular structure. Thephysical distance between neighboring light sources in the grid may befixed across the grid or may vary, for example as a function of lightoutput power required to provide the anti-fouling effect or as functionof the location of the lighting module on the protected surface (e.glocation on the hull of a ship). Advantages of providing atwo-dimensional grid of light sources include that the anti-foulinglight may be generated close to the areas to be protected withanti-fouling light illumination, and that it reduced losses in theoptical medium or light guide and that is increasing homogeneity of thelight distribution. Preferably, the anti-fouling light is generallyhomogeneously distributed across the emission surface; this reduces oreven prevents under-illuminated areas, where fouling may otherwise takeplace, while at the same time reducing or preventing energy waste byover-illumination of other areas with more light than needed foranti-fouling.

In preferred embodiments, the light sources are UV LEDs. The at leastone UV LED or the grid of UV LEDs may be encapsulated in a liquid-tightencapsulation. In embodiments the at least one UV LED or the grid of UVLEDs may be embedded in the optical medium. A plurality of UV LEDs maybe organised in grid and electrically connected in a series/parallelchicken-wire structure (as will be explained later). The LEDs and thechicken-wire connections may be encapsulated in a light-transmissivecoating and attached to the optical medium or directly embed in theoptical medium. In other embodiments the grid of UV LEDs may becomprised in a layer of electronic textile which is embedded in a resinstructure. In some embodiments the UV LEDs may be packaged LEDs, inwhich case they already may include an optical element to distribute thelight emitted from the LED package across a wide emission angle. Inother embodiment the UV LEDs may be LED dies, typically not comprisingoptical elements but being significantly thinner than packaged LEDs. Asan example, LED dies could be picked and placed onto a surface of theoptical medium (preferably the application surface, but the emissionsurface would do as well because of the small size of the componentswhich will nearly not interfering with the light emission function ofsaid surface), electrical wired via printing of conductive paste andfinally the LED dies and wiring could be encapsulated with a thinlayer/coating of the optical medium or any other backing layer forapplying the lighting module to the protected surface. Variousembodiments of embedded light sources allow the presented anti-foulingtechnology to be commercialized as a foil for applying on the hull ofships.

A system for anti-fouling of a protected surface may comprise aplurality of lighting modules as disclosed herein for arranging on theprotected surface so as to provide anti-fouling light over substantiallythe entire area of the protected surface.

Silicone materials can provide optical transmission for UV light withlittle loss compared to other materials. This is in particular the casefor shorter wavelength light, e.g. UV light with wavelengths below 300nm. A particularly efficient group of silicone materials is, or at leastcomprises, so-called methyl silicones, according to the general chemicalformula CH₃[Si(CH₃)₂O]_(n)Si(CH₃)₃, with “n” indicating any suitableintegral, as customary in organic chemistry. This type of siliconematerials happens to exhibit excellent UV transmission properties withlittle losses, at least compared to other silicone materials. Further,silicone materials are flexible and resilient so that they are robust,durable and capable of withstanding compression such as due to bumps,collisions etc of objects against the surface, e.g. bumping of a shipagainst a quai. Further, deformation of a ship's skin due to temperaturefluctuation, pounding by waves, ship's flexion over swell and heave etcmay be accommodated. Also, silicone materials can be applied and formedover surface structures: welds, rivets, etc. in or on the surface.Silicone materials also tend to adhere well to metals and paints so thata protective coating over the surface is formed. Visibly transparentsilicone materials enable reading of underlaying markings (e.g. paintedsymbols) covered by the silicone material. Further, they are generallywater repellent and may reduce friction and drag. On the one handsilicones can be made very smooth to reduce adherence of biofoulingorganisms to the layer and to reduce friction against flowing water,while on the other hand the material may be finely structured so as tomimic shark's skin which is also known to reduce friction in water atsufficient speed relative to the surrounding water. It is noted that astructured surface of an optical medium, in particular a light guide,can cause breaking conditions for total internal reflection andtherewith cause coupling out of light from the light guide that wasotherwise captured within and transmitted with total internalreflection. Thus, coupling out of light can be localised reliably.

UV grade silica has very low absorption for UV light and thus is verywell suitable as optical medium and light guide material. Relativelylarge objects may be made from using plural relatively small pieces orportions of UV grade silica together and/or so-called “fused silica”,while retaining the UV-transmissive properties also for the largerobject. Silica portions embedded in silicone material protect the silicamaterial. In such combination the silica portions may provide UVtransparent scatterers in an otherwise silicone material optical mediumfor (re-)distribution of the light trough the optical medium and/or forfacilitating outcoupling of the light from a light guide. Also, silicaparticles and/or particles of other hard, UV translucent material mayfortify the silicone material. In particular flake-shaped silicaparticles may be used, also in high density, of up to 50%, 70% or evenhigher percentages of silica in silicone material may provides a stronglayer that can resist impacts. It is considered that at least a part ofthe optical medium or light guide may be provided with a spatiallyvarying density of UV grade silica particles, in particular flakes, atleast partly embedded in a silicone material, e.g. to vary opticaland/or structural properties. Here, “flakes” denote objects having sizesin three cartesian directions, wherein two of the three sizes maymutually differ, however, each being significantly larger, e.g. a factor10, 20, or significanly more, e.g. factors of 100's, than the thirdsize.

In embodiments, in parts of the optical medium close to the emissionsurface for emitting the anti-fouling light from the optical medium, thedensity of the UV grade silica particles in the silicone material mayincrease from within the optical medium towards the emission surface ofthe optical medium, so that at or near the emission surface a relativelyhigh density of silica particles is provided. Although more or lessspherical and/or random-shaped particles may be used, silica flakes ofsub-millimeter length scales, e.g. with typical sizes down to a fewmicrometers, may be arranged so close together that under the influenceof very local forces, such as a point-impacts from sharp-tipped objects,and/or localised impacts from blunt objects, including scratches, tearsetc, the flakes may have some, if only little, freedom of movement inthe flexible silicone that they can slightly rearrange themselves,dissipating the impact energy and reducing damage to the light guide asa whole. Thus, a balance of properties can be struck that results inboth a robust and a somewhat deformable layer, yet also providing thedesired optical qualities. In an embodiment the proportion of siliconematerial in the optical medium varies gradually from about 100% (i.e.substantially pure silicone material) to below about 5% (mostly silica)from one side of the optical medium to an opposite side.

It is noted that particles, in particular flake-shaped particles, ofother material than silica may be used, e.g. glass or mica. Such othermaterials may also serve as scatterers for the anti-fouling light.Mixtures of particles of different materials may also be provided, whichmay comprise mixtures of translucent, opaque and/or optically activeparticles. Compositions of such mixtures may vary across the lightguide, e.g. to adjust transmittivity of the light guide for theanti-fouling light, in particular if in some portions relatively largeamounts of poorly-transmitting particles are used.

For manufacturing the optical medium, a series of layers of siliconematerial may be formed, each possibly having a different compositionwith regard to the amount and/or density of silica particles. The layersmay be very thin and at least some may be applied with a wet-on-wettechnique, i.e. providing the silicone material to the layer in liquidor gelatinous form that should harden to the desired layer, but whereina subsequent layer is applied to an earlier layer before the earlierlayer has fully hardened. Thus, a good adhesion between the layers ispromoted and in the final product different layers may be hardly to notdiscernible and a gradual change in composition may be achieved.Different layers may suitably be formed and/or applied by spraying ofthe layer material. A layered material may be formed to any suitablethickness with good quality control. Note that the optical medium, whichconstitutes a substantial part of the lighting module's surface, may beattached to the protected surface in any suitable way, including gluing.Silicone materials tend to exhibit strong adhesion to ceramic, glassyand metallic materials and spraying or smearing silicone material istherefore a very suitable manner of forming and attaching the opticalmedium to a substrate. A sprayed and/or smeared optical medium can alsoreadily be made in different desired shapes, e.g. following a waterline, specific markings and/or surface shapes. A layering technique mayalso facilitate orienting particles in the silicone material, e.g.arranging flakes generally parallel to the direction of expansion of thelayer and the surface coated with the layer.

In another aspect of the lighting module, the optical medium comprisesspaces, e.g. channels which are filled with gas and/or clear liquid,e.g. water, for guiding the light therethrough and an associated methodcomprises distributing at least part of the light through such spaces inan optical medium. It is found that optical transmission for UV lightthrough gaseous matter, in particular air, is generally significantlybetter than transmission of the light through a solid material whichmay, even if found translucent or transparent by some, exhibitabsorption losses of up to several percents per millimeter. Clear liquidprovides little scattering, may well transport UV light and may alsoprovide structural robustness of cavities in the optical medium comparedto filling the spaces with gas. Water, most notably fresh water, hasbeen found to have a relatively high and suitable UV transmittivity.Contamination and/or UV absorption may be also and/or further reduced ifdistilled, deionised and/or otherwise purified water is used. Hence, itis considered particularly beneficial to transmit the light through agas- and/or liquid-filled space. For distribution of the light acrossthe protected surface, the gas- and/or liquid-filled space shouldpreferably be well defined and channels may be provided in a opticalmedium. Light that eventually strikes walls of the channels can enterthe optical medium and be emitted from the optical medium in a directionfrom the protected surface and into the liquid environment to providethe anti-fouling light. An optical medium in which the air channels aredefined that is itself well transparent to the anti-fouling lightfurther assures that if the optical medium would leak and the liquidmedium enters the optical medium, generated anti-fouling light wouldstill be appropriately transmitted through the optical medium. Channelsmay comprise varying diameter. Localised channel portions or pockets maybe provided by wall portions defining and encapsulating separate volumes(much) bigger than the respective wall portions' sizes and/orthicknesses, e.g. similar to the packaging product sold under the brandname “Bubble Wrap”.

In a particular embodiment, such gas-containing optical medium comprisesa silicone material defining the gas and/or liquid-filled channelsand/or other spaces; silicone materials may well be shaped to defineintricate structures. Further advantages of silicone materials, with orwithout additional objects such as silica particles have been set outabove.

In an embodiment, the channels and/or other spaces are provided byforming two opposing layers of silicone material kept separated atdesired distances with wall portions and/or pillars of silicone materialcreating a distance, e.g. an air gap between the layers. Such wallportions and/or pillars may serve as scattering centres for(re-)distributing the light through (the channels in) the optical mediumand/or for guiding light from the gas- and/or liquid filled space(s)into the silicone material. This facilitates localising emission of thelight from the optical medium into the liquid environment where theanti-fouling light is to be put to use.

At least part of the anti-fouling light emitted by the one or more lightsources may be spread in a direction having a component substantiallyparallel to the protected surface, or substantially parallel to theapplication surface of the foil when the light moduled is provided as afoil. This facilitates distributing the light over significant distancesalong the protected surface, or the application surface of the foil,which assists in obtaining a suitable intensity distribution of theanti-fouling light.

A wavelength conversion material may be comprised in the optical mediumand at least part of the anti-fouling light may be generated byphoto-exciting the wavelength conversion material with light having afirst wavelength causing the wavelength conversion material to emit theanti-fouling light at another wavelength. The wavelength conversionmaterial may be provided as an upconversion phosphor, quantum dots,nonlinear media such as one or more photonic crystal fibers etc. Sinceabsorption and/or scattering losses in the optical medium for light ofdifferent, mostly longer, wavelengths than UV light tend to be lesspronounced in optical media, it may be more energy-efficient to generatenon-UV light and transmit that through the optical medium and togenerate UV anti-fouling light at or near the desired location of usethereof (i.e. emission form the surface into the liquid environment).Also, or alternatively, the at least one light source may comprise atleast one of an LED or OLED, a DBD lamp and/or a metal vapour lamp (e.g.low pressure mercury vapour lamp). Suitable anti-fouling light is in thewavelength range of UV or blue light from about 220 nm to about 420 nm,in particular at wavelengths shorter than about 300 nm, e.g. from about240 nm to about 280 nm.

In embodiments, the optical medium comprises a light spreader arrangedin front of the at least one light source for generating anti-foulinglight for spreading at least part of the anti-fouling light emitted bythe at least one light source in a direction having a componentsubstantially parallel to the protected surface. An example of a lightspreader may be a ‘opposite’ cone arranged in the optical medium andposition opposite the at least one light source, where the opposite conehas a surface area with a 45° angle perpendicular to the protectedsurface for reflecting light emitted by the light source perpendicularto said surface in an a direction substantially parallel to saidsurface. In embodiments the optical medium comprises a light guidearranged in front of the at least one light source for generating theanti-fouling light, the light guide having a light coupling-in surfacefor coupling in the anti-fouling light from the at least one lightsource and a light coupling-out surface for coupling-out theanti-fouling light in a direction away from the protected surface; thelight guide comprising a light guide material having a refractive indexhigher than the refractive index of the liquid environment such that atleast part of the anti-fouling light is propagated through the lightguide via total internal reflection in a direction substantiallyparallel to the protected surface before being out-coupled at theout-coupling surface. Some embodiment may comprise an optical mediumwhich combines a light spreader and a light guide, or integrated lightspreading features with light guiding features into the optical medium.In embodiments, the light spreader and/or light guide is coated onto theprotected surface. In other embodiments, the light spreader and/or lightguide is provided in the form factor of a foil for applying onto aprotected surface.

An embodiment of a system for preventing fouling may comprise:

-   -   a series of UV LEDs for generating anti-fouling light;    -   a light spreader for spreading the anti-fouling light from the        LED point sources across the protected surface; and    -   a light guide for further guiding/spreading the anti-fouling        light can be spread across the surface, the light guide        comprising a tin layer of silicone material transparent to UV        light, with or without silica particles or one or more silica        covered portions.

When substantially the entire protected surface is covered with ananti-fouling light emitting optical medium, it substantially reduces thegrowth of micro-organisms on this medium. As the micro-organisms arekilled on the emission surface of the optical medium, the hull iscontinuously cleaned through the water flow along the hull whichtransports the debris away from the ship and micro-organisms do notstand a chance of fouling on the hull.

It is an advantage of the presently provided solutions that themicro-organisms are not killed after having adhered and rooted on thefouling surface, as is the case for known poison dispersing coatings,but that the rooting of micro-organisms on the fouling surface isprevented. It is more efficient to actively kill micro-organism rightbefore or just after they contact the fouling surface, compared to alight treatment to remove existing fouling with large micro-organismstructures. The effect may be similar to the effect created by usingnano-surfaces that are that smooth that micro-organism cannot adhere toit.

Because the low amount of light energy required for killing themicro-organism in the initial rooting stage, the system may be operatedto continuously provide an anti-fouling light across a large surfacewithout extreme power requirements.

A grid of LEDs creating a lighting surface may be provided with energyharvesting means such as for example embedded solar cells, smallturbines operating in the water, piezoelectric elements operating onpressure waves, etc.

Some advantages of the presently provided technology include theretention of clean surface, Reduction of the cost of corrosiontreatment, reduced fuel consumption for ships, reduced maintenance timefor hulls, educed CO₂ emission, reduce the use of toxic substances inthe environment, etc. A substantially planar and smooth light emissionsurface further has the advantage of not adding drag by itself and caneven further reducing drag by burying existing uneveness (rivets, welds,etc.) of the protected surface underneath the optical medium.

The features disclosed in the context of a lighting module described inthe present disclosure may also have a corresponding process step in themethod for anti-fouling of a protected surface and vice versa, withoutexplicitely being mentioned in the description. Corresponding featureswill generally produce the same technical effect.

The disclosed method and lighting module can be applied to preventfouling on hulls of ships, but they are applicable to all marine objectsincluding stationary (pipes, marine stations etc.) and/or moving marineobjects (submarines etc.). The disclosed anti-fouling solution may alsobe applied to objects operating in waterways, canals or lakes and forexample also to aquariums.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a ships hull suffering from fouling;

FIG. 2 is a graph showing UV Light Threshold Limited Values (TLV)according to the American Congress of Governmental and IndustrialHygienist's (ACGIH);

FIG. 3 is a graph showing a germicidal action spectrum for differentbiological materials as a function of light wavelength;

FIG. 4 is a graph showing a transmission spectrum for different types ofglass;

FIG. 5 is a bar graph showing the relative spectral power distributionof typical Philips low pressure tubular fluorescent ultraviolet (TUV)lamps;

FIG. 6 is a bar graph showing the relative spectral power distributionof Philips medium pressure discharge lamps (HOK and HTK types);

FIG. 7 is a schematic cross section view of a light module with a lightguide;

FIG. 8 shows a general concept of light guiding used in embodiments;

FIGS. 9(a)-9(b) show a realised planar light guide embodiment;

FIGS. 10(a)-10(b) show wedge shaped light guide embodiments;

FIGS. 11(a)-11(b) show direct-lit light guide embodiments;

FIG. 12 shows an embodiment comprising a redistribution reflector and awavelength conversion material;

FIG. 13 shows a light guide comprising gas-filled channels;

FIG. 14 shows an embodiment comprising distributed embedded flakes.

FIG. 15 shows an embodiment of a chicken-wire grid.

DETAILED DESCRIPTION OF EMBODIMENTS

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; thedisclosure is not limited to the disclosed embodiments. It is furthernoted that the drawings are schematic, not necessarily to scale and thatdetails that are not required for understanding the present inventionmay have been omitted. The terms “upward”, “downward”, “below”, “above”,and the like relate to the embodiments as oriented in the drawings,unless otherwise specified. Further, elements that are at leastsubstantially identical or that perform an at least substantiallyidentical function are denoted by the same numeral.

FIG. 7 shows as a basic embodiment a cross section of a lighting module1 comprising a plurality of light sources 3 (here: side-emitting LEDs,wherein the light is emitted primarily from the side of the LED, andmore or less parallel to the surface) encapsulated in a liquid-tightoptical medium 5 to guide at least part of the light 9 emitted from thelight sources 5 via total internal reflection through the opticalmedium, which optical medium is further provided with optical structures7 to scatter light 9 and guided the light 9 out of the optical medium 5towards an object 11 to be targeted with the light (a biofoulingorganism). The optical medium 5 generally extends in two dimensionssignificantly further than in the third dimension so that atwo-dimensional-like object is provided. Optical structures 7 to scatterlight 9 may be spread in one or more portions of the optical mediummaterial, possibly throughout all of it, wherein in such portions thedistribion may be generally homogeneous or localised. Scattering centreswith different structural properties may be combined to provide, besidesoptical, also structural characteristics, such as resistance to wearand/or impact. Suitable scatterers comprise opaque objects but largelytranslucent objects may be used as well, e.g. small air bubbles, glassand/or silica; a requirement is merely that a change in refractive indexoccurs for the wavelength(s) used.

The principle of light guiding and spreading light over a surface iswell-known and widely applied in various fields. Here, the principle isapplied to UV light for the purpose of anti-fouling. It is noted thatthe idea of making a surface, e.g. the hull of a ship self-lit with UVis a clearly different solution than the current and well establishedanti-fouling solutions which rely on smooth coatings, chemicals,cleaning, software to control the ship speed, etc.

Total internal reflection is one way of transmitting light through anoptical medium, which is then often referred to as a light guide. Tomaintain the conditions for total internal reflection, the index ofrefraction of the light guide should be higher than that of thesurrounding medium. However, the use of (partly) reflecting coatings onthe light guide and/or use of the reflective properties of the protectedsurface, e.g. the hull of a ship, itself can also be used to establishthe conditions for guiding the light through the optical medium.

In some embodiments the optical medium may be positioned relative to theprotected surface, e.g. the hull of a ship, such that a small air gap isintroduced between the optical medium and the protected surface; UVlight may travel even better—with less absorption—in air than in anoptical medium, even when this optical medium is designed as a lightguiding material. In other embodiments gas-filled channels, e.g. airchannels, may be formed within silicone material. An array of separategas-filled pockets may also be provided, e.g. in a regular pattern likea rectangular or honeycomb-pattern or in an irregular pattern. Insteadof gas (e.g. air) filling, channels and/or pockets may be at leastpartly filled with a UV-transmissive liquid, e.g. fresh and/or purifiedwater. In case a protected surface that is covered with such opticalmedium is subject to impact, e.g. a ship hitting a dockside, smallpockets may soften, redistribute the impact energy and hence protect thesurface, wherein liquid-filled pockets may be robuster under deformationthan air-pockets which may more easily burst open.

As most materials have a (very) limited transmittance for UV light, carehas to be taken in the design of the optical medium. A number ofspecific features and/or embodiments, which are dedicated for thispurpose are listed in the following:

-   -   A relatively fine pitch of low power LEDs can be chosen, to        minimize the distance light has to travel through the optical        medium.    -   A ‘hollow’ structure can be used, e.g. a silicone rubber mat        with spacers that keep it a small distance away from the        protected surface. This creates air ‘channels’, through which        the UV light can propagate with high efficiency (air is very        transparent for UV). Use of gas filled channels provided by such        structures allows distributing the UV light over significant        distances in a optical medium of material that would otherwise        absorb the UV light too strongly to be useful for anti-fouling.        Similarly, separate pockets may be formed.    -   A special material can be chosen with high UV transparency, like        certain silicones or UV grade (fused) silica. In embodiments,        this special material can be used only for creating channels for        the light to propagate the majority of the distance; a        cheaper/more sturdy material can be used for the rest of the        surface.

Further embodiments are disclosed in the accompanying drawings, whereina main issue is to illuminate a large surface with anti-fouling light,preferably UV light, yet using point light sources. A typical concern isspreading of the light from point sources to surface illumination. Inmore detail:

-   -   The protected surface area of a typical container ship is        ˜10.000 m².    -   A typical LED source has an area of ˜1 mm². This is 10¹⁰        smaller.    -   Taking the required power levels into account, about 10 LEDs per        m² may be required    -   This means light has to be spread from 1 LED over ˜1000 cm²    -   As another boundary condition is taken that the solution should        be thin (order of magnitude: 1 cm), e.g. for reasons such as:        -   To be able to add the solution as a ‘coating’ to a ship        -   To not increase drag due to an increased cross section size            of the ship        -   To keep (bulk) material costs limited.

The use of an optical medium, in particular a generally planar lightguide is therefore provided. Typical dimensions of a light guide are athickness of about 1 mm to about 10 mm. In the other directions, thereis no real limit to the size, from an optical point of view; inparticular not if plural light sources are provided so that decay oflight intensity throughout the light guide due to partial outcoupling oflight and possibly (absorption) losses are countered.

Here, it is considered that similar optical challenges apply as with thedesign of LCD TV backlights, although emission light intensityuniformity is less stringent in anti-fouling than with LCD TVbacklights. FIG. 8 shows a lighting module 1 with light sources 3 and alight guide 5 with an additional top layer 13. FIGS. 9A-9B showpractical examples of the principle illustrated in FIG. 8 and show alighting module 1 with LED sources 3 which are positioned along the edge15 of a light guide 5 and which inject light into the light guide 5. Apattern of scatterers e.g. white dots of paint, or small scratches/dentsextract the light in appropriate places, here generally uniform (FIG.9B), so that a desired, e.g. generally homogeneous, illuminationdistribution of the environment is achieved.

FIG. 10(a) shows a LCD TV backlight arrangement wherein a wedge-shapedlight guide 5(a) is employed wherein the light from a light source 3 isinjected into the light guide 5(a) from the side. The light guide 5(a)is arranged with a pattern of scattering objects 7, such as dots ofpaint or scratches, on a reflective substrate 17. A wedge shape causesmore of the light to be extracted towards the tip end. The prism sheets19 and LCD panel 21 that orient polarisation states of the light andgenerate visible light colours are feature that can be omitted in ananti-fouling context.

FIG. 10(b) shows another wedge shaped light guide 5(b) which is provideditself with a structured side so as to scatter and redistribute lightwithin and out of the light guide 5(b).

Both the plane light guide and wedge-shaped light guide share theprinciple of guiding light along a substantial distance substantiallyparallel to the emission surface. The alternatives shown in FIG. 11a-11(b) (see below) are known as a direct-lit optical medium; here one ormore LEDs and/or other light source(s) is present behind a screen e.g. adiffuser and emit light directly towards the object to be illuminated,e.g. a biofouling organism.

In a side-lit optical medium, often referred to as a light guide, suchas those shown in FIGS. 8-10(b) a side of the optical medium isilluminated from one or more light sources relatively strongly andfurther away from the light source(s) the light intensity within lightguide is generally more homogeneous, possibly governed by scatterers(FIGS. 9(a)-9(b)).

In short, a difference between side-lit or direct-lit concepts is thatin direct-lit situations the light travels no substantial distanceparallel to the emission surface. As a result, the light intensity isusually much higher directly in front of the light sources. No realdistribution of light is achieved. Thus, in a direct-lit solution alarger intensity variation may be expected between areas directly infront of the light source(s) and area aside thereof.

FIGS. 11(a) and 11(b) show lighting modules 101(a), 101(b) in crosssection view (cf FIG. 7) comprising light sources 3 and optical medium105 (a), 105(b) having an emission surface 23. The wavy line “I(a)” and“I(b)”, respectively, show the light intensity profile emitted from theemission surface and illustrate that a thicker optical medium 105(b)(FIG. 11(b)) will ‘automatically’ provide a better light uniformity onthe emission surface 23 than a thinner optical medium 105(a) (FIG.11(a)) of otherwise identical construction.

However, in the present case such relative intensity variations need notbe of much concern. Further, direct lit arrangements potentially alsohave capability of controlling local intensity variations, which mayalso be utilised for providing both temporal and spatial intensityvariations. Thus, the optical structure provided herewith is relativelysimple. As a rule of thumb, for a high level of emission lightintensity, the thickness of a optical medium in a direct-litconfiguration is generally about equal to the LED pitch. If the LEDpitch is 10 cm, this rule of thumb might lead to an optical medium thatis about 10 cm thickness, which is thicker than desired. However, thelight emission uniformity requirements for the presently intendedpurpose of anti-fouling do not have to meet ‘substantially uniformlighting’ requirements and hence a thinner layer may be used incombination with such LED pitch.

Additional ideas and solutions exist to obtain a better uniformity in athinner optical structure, such as introduction of scatters and/orreflectors or other light spreaders directly in front of one or morelight sources.

FIG. 12 shows (left hand side) inclusion of a light spreader in the formof a reflective cone 25 in the optical medium 5 with an apex towards thelight source 3. This directs the light 9 in a direction having acomponent substantially parallel to the surface 27 to be protectedagainst fouling. If the cone 25 is not fully reflective nor opaque, somelight from the light source will pass through it and creation of shadowsleading to reduced or ineffective anti-fouling is prevented.

Further, FIG. 12 shows (right hand side) a wavelength conversionmaterial which is comprised in the optical medium 5. The illustratedembodiment is configured to generate at least part of the anti-foulinglight by photo-exciting the wavelength conversion material with lightfrom a light source 30 with light 31 having a first wavelength causingthe wavelength conversion material to emit anti-fouling light 9 atanother wavelength from the optical medium 5 into the environment E. Thedistribution of wavelength conversion material in optical medium 5 maybe spatially varying, e.g. in accordance with (expected) intensitydistributions of (different wavelengths of) light in the optical medium5.

FIG. 13 shows an optical medium 205 comprising a first layer 233, asecond layer 235 with a plurality of walls 237 and pillars 238 inbetween separating the first and second layers 233, 235 and creatinggas-filled channels 239. The optical medium 205 may be used just as anyof the other optical mediums shown herein.

FIG. 14 shows a portion of an object 300 to be protected againstbiofouling, comprising an object surface 301, e.g. a ship hull, providedwith an optical medium 302 comprising embedded flake-shaped particles303. (In the drawing, the light sources are omitted.) The flakes 303 aredistributed generally parallel to each other and with increasing densityfrom the object surface 301 outwards to an emission surface 304.

FIG. 15 shows a chicken-wire embodiment where UV LEDs 3 are arranged ina grid and connected in a series of parallel connections. The LEDs canbe mounted at the nodes as shown in bottom left of FIG. 15 eitherthrough soldering, glueing or any other known electrical connectiontechnique for connecting the LEDs to the chicken wires 4. One or moreLEDs can be placed at each node. DC or AC driving can be implemented. Incase of DC, the LEDs are mounted as shown at the bottom right (a) ofFIG. 15. If AC is used, then a couple of LEDs in anti parallelconfiguration is used as shown at the bottom right (b) of FIG. 15. Theperson skilled in the art knows that at each node more than one coupleof LEDs in anti parallel configuration can be used. The actual size ofthe chicken-wire grid and the distance between UV LEDs in the grid canbe adjusted by stretching the harmonica structure. The chicken-wire gridmay be embed in an optical medium wherein optionally a parallel grid ofscattering features are provided as illustrated in FIG. 12.

Besides anti-fouling application of hulls of ships, the followingalternative applications and embodiments are envisioned:

-   -   The disclosure can be applied to a wide variety of fields.        Almost any object coming into contact with natural water, will        over time be subject to biofouling. This can hinder e.g. water        inlets of desalination plants, block pipes of pumping stations,        or even cover the walls and bottom of an outdoor pool. All of        these applications would benefit from the presently provided        method, lighting modules and/or system, i.e. an effective thin        additional surface layer, which prevents biofouling on the        entire surface area.    -   Although UV light is the preferred solution, other wavelengths        are envisaged as well. Non-UV light (visible light) is also        effective against biofouling. Typical micro-organisms are less        sensitive to non-UV light than to UV light, but a much higher        dose can be generated in the visible spectrum per unit input        power to the light sources.    -   UV LEDs are an ideal source for thin light emitting surfaces.        However, UV sources other than LEDs can also be used, such as        low pressure mercury vapour lamps. The form factor of these        light sources are quite different; mainly the source is much        bigger. This results in different optical designs, to        ‘distribute’ all the light from a single source over a large        area. The concept of light guiding as discussed herein does not        change though. Further, a significant contribution of light in        desired wavelengths and/or wavelength combinations may be        produced.

Instead of using a thin layer that emits UV light outward in a directionaway from the protected surface in order to avoid bio-fouling,biofouling could potentially also be removed by applying UV light fromthe outside in the direction of the protected surface. E.g. shining a UVlight onto a hull or surface comprising a suitable optical medium asdescribed. Thus, a single optical medium emitting anti-fouling light indirections to and away from protected surfaces may be even moreefficient.

The concepts are not restricted to the above described embodiments whichcan be varied in a number of ways within the scope of the claims. Forinstance, using light, in particular UV light as an anti-biofoulingmeans can provide an interesting opportunity in other fields. It isunique in the sense that continuous “24/7” ‘protection’ can be provided,over a large area. The application is especially interesting for thehull of ships, but can also be applied in swimming pools, watertreatment plants, etc. Instead of water, biofouling may occur and betreated in other liquid environments, e.g. oils, brines and/or liquidsin other environments including food industry.

Elements and aspects discussed for or in relation with a particularembodiment may be suitably combined with elements and aspects of otherembodiments, unless explicitly stated otherwise.

The invention claimed is:
 1. A method of anti-fouling of a protectedsurface comprising, while the protected surface is at least partiallysubmersed in a liquid environment, in particular an aqueous or oilyenvironment: providing an anti-fouling light; providing an opticalmedium in close proximity to the protected surface, the optical mediumhaving an emission surface and a plurality of optical structurescomprising light spreaders arranged opposite the emission surface;distributing at least part of the anti-fouling light through the opticalmedium in a direction substantially parallel to the protected surface;and emitting the anti-fouling light from the emission surface of theoptical medium in a direction away from the protected surface; whereinthe anti-fouling light is emitted by at least one light sourcecomprising, at least one UV LED embedded in the optical medium and thatthe emission surface is a substantially planar surface masking orobscuring a thickness of the at least one light source and wiringconnections embedded in a lighting module.
 2. The method of claim 1,wherein the anti-fouling light is emitted from a plurality of lightsources arranged in a two-dimensional grid onto or in close proximitythe protected surface.
 3. The method of claim 1, wherein a wavelengthconversion material is included in the optical medium and at least partof the anti-fouling light is generated by photo-exciting the wavelengthconversion material with light having a first wavelength causing thewavelength conversion material to emit the anti-fouling light at anotherwavelength.
 4. The method of claim 1, wherein the optical mediumcomprises a silicone material, selected from a group comprising at leastone of: methyl-silicones, and UV grade silica material, and wherein theoptical medium is one of a light guide, a light spreader or acombination of a light guide and a light spreader.
 5. The method ofclaim 1, comprising distributing at least part of the anti-fouling lightthrough one of: spaces, and channels, in the optical medium, which arefilled with at least one of: a gas and clear liquid.
 6. The method ofclaim 1, comprising: providing at least a part of the optical mediumwith a spatially varying density of UV translucent particles comprisingUV grade silica particles formed as flakes, at least partly embedded ina silicone material, wherein a density of the UV grade silica particlesincreases from within the optical medium towards the emission surface ofthe optical medium.
 7. A ship comprising at least one lighting moduleaccording claim
 6. 8. A lighting module for anti-fouling of a protectedsurface comprising: at least one light source for generating ananti-fouling light; an optical medium for distributing at least part ofthe anti-fouling light through the optical medium in a directionsubstantially parallel to the protected surface, the optical mediumcomprising: an emission surface for emitting the distributedanti-fouling light in a direction away from the protected surface and aplurality of optical structures comprising light spreaders arrangedopposite the emission surface, wherein the at least one light sourcecomprises at least one UV LED embedded in the optical medium and thatthe emission surface is a substantially planar surface masking orobscuring a thickness of the at least one light source and wiringconnections embedded in the lighting module.
 9. The lighting module ofclaim 8 comprising: a plurality of light sources for generating theanti-fouling light, where the plurality of light sources are arranged ina two-dimensional grid, in particular in a series of parallelconnections in a chicken-wire structure.
 10. The lighting module ofclaim 8, wherein the at least one light source or the two-dimensionalgrid of the plurality of light sources is encapsulated in a liquid-tightencapsulation.
 11. The lighting module of claim 8, wherein the opticalmedium comprises a silicone material, selected from a group comprisingat least one of: methyl-silicones, and UV grade silica material, andwherein the optical medium is one of a light guide, a light spreader ora combination of a light guide and a light spreader.
 12. The lightingmodule of claim 8 wherein the optical medium comprises one of: spacesand channels, filled with one of: a gas and a clear water.
 13. Thelighting module of claim 8, wherein the at least one light source is atleast one of a Light Emitting Diode or an Organic Light Emitting Diode(LED or OLED), wherein the at least one light source is configured toemit anti-fouling light in a wavelength range of UV light of about 240nm to about 280 nm.
 14. The lighting module of claim 8, wherein at leasta part of the optical medium comprises: a spatially varying density ofUV translucent silica particles shaped as flakes, at least partlyembedded in a silicone material wherein the density of the UV gradesilica particles increases from within the optical medium towards theemission surface of the optical medium in at least part of the opticalmedium.
 15. A ship according to claim 14 wherein the hull is theprotected surface.
 16. The lighting module of claim 8, wherein thelighting module is shaped as a tile or an elongated strip.
 17. A systemfor anti-fouling of a protected surface, comprising: a plurality oflighting modules according to claim 8 where the plurality of lightingmodules are arranged on the protected surface so as to provideanti-fouling light over substantially the entire area of the protectedsurface.
 18. The lighting module of claim 8, comprising: a wavelengthconversion material is within in the optical medium and configured to:generate at least part of the anti-fouling light by photo-exciting thewavelength conversion material with light having a first wavelengthcausing the wavelength conversion material to emit the anti-foulinglight at another wavelength.